The present invention relates to x-ray computed tomography using helical scanning and specifically to a reconstruction technique for helical scanning providing improved slice interpolation.
X-ray computed tomography is a well-known procedure for creating cross-sectional images from computer processed x-ray projections taken along the plane of the cross section. In a typical CT machine, an x-ray tube is mounted on a rotatable gantry to project the fan beam of x-rays at a patient through a xe2x80x9cslicexe2x80x9d from a variety of angles. The x-rays are received after passing through the patient by a multi-element detector to provide a measurement of x-ray attenuation along a variety of rays of the fan beam. The attenuation signals from the elements of the multi-element detector are sampled and digitized by a data acquisition system.
Digitized projections collected at a range of angles about the patient, typically no less than 180xc2x0 plus half the fan beam angle, are collected in as a xe2x80x9ctomographic projection setxe2x80x9d and reconstructed according to well known techniques in the art, such as filtered back projection, into an image of a cross section of the patient along that slice. Volumetric images may be obtained by taking a series of adjacent slices to permit three-dimensional modeling of structures within the patient or to permit arbitrary changes in the cross-sections obtained.
The time required to obtain one or more tomographic projection sets may be shortened by simultaneously rotating the gantry and moving of the patient along the gantry""s axis of rotation. This technique is termed xe2x80x9chelical scanningxe2x80x9d because the path of the x-rays follows a helix about the patient.
In helical scanning, the projections of the tomographic projection set do not lie in a single plane, a condition necessary for tomographic reconstruction. Accordingly, the projections may be interpolated to a common and arbitrarily chosen xe2x80x9cslice planexe2x80x9d. Actual projections taken at common angles but displaced on either side of the slice plane later or earlier in the helical scan are used as end points in the interpolation. The interpolated data may then be reconstructed according to conventional techniques.
Additional increases in the speed of acquisition may be obtained by providing the multi-element x-ray detector with several rows displaced from one another along the axis of rotation of the gantry. In this way, multiple tomographic projections can be acquired at a given time. Using such a multi-row x-ray detector, interpolation to a single image plane may be accomplished by using projections acquired by different rows within the detector at a single gantry position.
At certain times, it may be desirable to acquire tomographic projections of extremely thin slices. The slice thickness can be controlled by collimating the fan beam to an arbitrary axial dimension limited only by minimum scan speeds and acceptable signal to noise ratio. Importantly, the axial dimension of the fan beam may be less than the axial dimension of the multi-element x-ray detector or a single row of a multi-row x-ray detector. This preserves the ability of the CT machine to also image thicker slides.
Although adjusting the collimator to obtain thin slices, each narrower than a row of the x-ray detector, works well for non-helical scanning, when this technique is used in conventional helical scanning, unacceptable image artifacts are produced.
The present inventors have recognized that producing thin slices by collimation of the fan beam can produce a variation in effective x-ray sensitivity across the axial dimension of the detector rows. This variation is caused principally by a penumbra at the edges of the collimated x-ray beam. This variation, in turn, introduces a nonlinearity to the interpolation process used in helical scanning, which, if uncorrected, can introduce artifacts to the image.
Accordingly, the present invention provides a correcting weighting to the interpolation of helically acquired data to a single slice plane. Generally the weighting considers the degree of overlap of the desired slice with asymmetric detector profiles of the two detectors. The weighting compensates for nonlinear variations in the overlapped area as a function of slice location.
Specifically, the present invention provides a method of data acquisition for a computed tomography machine having an opposed x-ray source and a multi-row x-ray detector mounted on a gantry for helical rotation about a patient along a longitudinal axis. Longitudinal variation in x-ray detection across the rows of the multi-row x-ray detector are determined and the detector is used to acquire a set of tomographic projection data at angles about the patient over a helical path. A slice having a location and thickness for tomographic reconstruction is selected. Then, for each of the rows, the data of the detector elements of the rows are weighted according to the modeled variation in x-ray detection of the row and according to an overlap of the slice with the row. The weighted data is reconstructed.
In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.